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#38
July 17th 03, 02:20 AM
 Roy Lewallen Posts: n/a

Dr. Slick wrote:
Roy Lewallen wrote in message ...

What if the antenna was a wire shorting the transmission line?
Then there would be very little radiated power, and a lot of reflected
power.

Yes, there would. But every watt delivered to that wire would be either

Actually mostly reflected back to the source, so not radiated or
dissipated assuming ideal lossless transmission lines.

No. "Reflected" power isn't delivered to the wire. It's an analytical
function that exists only on the feedline. If the feedline has no loss,
the same amount of power entering the line exits the line. You can have
any amount of "reflected power" you want by simply changing the
characteristic impedance of the line -- with no effect on the power
either exiting or leaving the line.

. . .

Except in the case of free space, with a given permeability and
permittivity, you have the impedance of free space, which doesn't need
a transmission line.

Sorry, I can't make any sense out of that.

Well, my point was that you don't need a transmission line for
free space because otherwise it wouldn't be wireless. But your above
point is well taken, that there is no current flowing in free space,
in an expanding EM wave, while there definitely is current in a
transmission line.

Yes, that's right. But if you want to dig deeper, you'll find that a
"displacement current" can be mathematically described which
conveniently accounts for some electromagnetic phenomena. It is, though,
a different critter from the conducted current on a transmission line.

An antenna can reasonably be viewed as a transducer. It converts the
electrical energy entering it into electromagnetic energy -- fields. As
is the case for any transducer, the stuff coming out is different than
the stuff going in. Think in terms of an audio speaker, which converts
electrical energy into sound waves, and you'll be on the right track.

. . .

What's wrong with thinking of an antenna as a type of series
Inductor, with a distributed shunt capacitance, that can be thought of
as a type of distributed "L" matching network that transforms from 50
Ohms to 377?

Because that's not what it does, and thinking of it that way leads you
to impossible conclusions. The antenna is converting power to E and H
fields. The ratio of E to H, or the terminal V to I are immaterial to
the conversion process. You're continuing to be suckered into thinking
that because the ratio of E to H in free space has the dimensions of
ohms that it's the same thing as the ratio of V to I in a circuit. It
isn't. A strand of spaghetti one foot long isn't the same thing as a one
foot stick of licorice, just because the unit of each is a foot.

But an antenna must be performing some sort of transformer action.
If you were designing an antenna to radiate underwater, or though
Jell-o, or any other medium of a different dielectric constant than
free space, you would have to change it's geometry. Even if it is E
to H, and not V to I.

Not transformer, transducer. More like a speaker than a megaphone.

If an antenna is not a transformer of some type, then why is it
affected by it's surroundings so much?

Egad, how can I answer that? If a fish isn't a transformer, then why is
it affected by its surroundings so much? If an air variable capacitor
isn't a transformer, then why is it affected by its surroundings so
much? What does sensitivity to surroundings have to do with being a
transformer?

They obviously are, just like
the primary's impedance is affected by what the secondary sees in a
transformer. Certainly having lots of metal in close proximity will
affect the impedance of your antenna.

It is true that the equations describing coupling between two antenna
elements are the same as the for the coupling between windings of a
transformer. But a single antenna element isn't a transformer any more
than a single inductor is a transformer. When you apply V and I to an
antenna, it creates E and H fields. When you apply V and I to an
inductor, it creates E and H fields. Both the antenna and inductor are
acting as transducers, converting the form of the applied energy. If you
put a secondary winding in the field of the primary inductor, the field
induces a voltage in the secondary. If (and only if) the secondary is
connected to a load, causing current to flow in it, that current
produces a field which couples back to the primary, altering its
current. Coupled antennas, or an antenna coupled to any other conductor,
work the same way -- although localized currents can flow in the absence
of an intentional load if the antenna is a reasonable fraction of a
wavelength long.

But the single antenna isn't a transformer, any more than the single
inductor is. Each is a transducer, and the secondary winding, or coupled
conductor, is another transducer.

This is related to how you need to bend the ground radials of a
1/4 WL vertical whip at 45 deg angles down, to get the input impedance
closer to 50 Ohms (as opposed to 36 Ohms or something like that if you
leave them horizontal).

There are a number of ways in which an antenna and transmission line are
similar. But don't take the analogy too far. Start with a quarter
wavelength transmission line, start splitting the conductors apart until
they're opposed like a dipole, and tell me how you've ended up with an
input impedance of 73 ohms.

Well, I'm not sure, but you would start off at an open, which
would be transformed to a virtual short. But from there, it sounds
like a complex mathematical derivation to get the 73 Ohms.

And it's really, really tough and requires some *really* creative (read:
bogus) math to derive it from simply transmission line phenomena.

My point is that the 73 Ohms is dependant on the dipole's
surroundings, depending on how far from the ground and such, so it is
a transformer of some sort.

Not by itself it isn't. But if you can make a transformer by putting two
antenna elements close together -- put V and I into one and extract it
in a different ratio from the other. It's going to be a pretty lossy
transformer, though, due to energy lost to radiation. (You'll find extra
resistance at the "primary" feedpoint that'll nicely account for this.)

There are plenty of texts you can read, on all different levels, if
you're really interested in learning about antennas, fields, and waves.

Roy Lewallen, W7EL

Which one's can you recommend?

One of my favorites is King, Mimno, and Wing, _Transmission Lines,
Antennas, and Waveguides_, and that's probably the one I'd choose if I
had to select just one. It was reprinted as a paperback by Dover in
1965, and the paperback be found as a used book pretty readily and
inexpensively. Kraus' _Antennas_ is my favorite text on antennas, and is
certainly one of the most, if not the most, highly regarded. It's now in
its third edition, and you can often find used copies of earlier
editions at reasonable prices. For transmission lines, there's an
excellent treatment in Johnson's _Transmission Lines and Networks_. I
refer to Kraus' _Electromagnetics_ particularly when dealing with waves
in space. And Holt's _Introduction to Magnetic Fields and Waves_ is
pretty good for both.

There are a lot of others, each with its strong and weak points. But you
can't go wrong with these.

Roy Lewallen, W7EL